Vehicle and method of diagnosing battery condition of same

ABSTRACT

A method for diagnosing a battery may include applying a generally constant current to the battery for a period of time, measuring a response voltage of the battery to the applied current, determining battery impedance parameters based on at least one of the applied current, period of time, and response voltage, and determining a degradation condition of the battery based on the battery impedance parameters.

BACKGROUND

Plug-in hybrid electric vehicles may include a high voltage traction battery and a low voltage auxiliary battery. Each of the batteries may be charged with energy from an electrical grid.

SUMMARY

A generally constant current may be applied to a battery for a period of time. A response voltage of the battery to the applied current may be measured. Battery impedance parameters may be determined based on at least one of the applied current, period of time, and response voltage. A condition of the battery may be determined based on the battery impedance parameters.

While example embodiments in accordance with the invention are illustrated and disclosed, such disclosure should not be construed to limit the invention. It is anticipated that various modifications and alternative designs may be made without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of an alternatively powered vehicle.

FIG. 2 is a schematic diagram of an equivalent circuit representing cells of the auxiliary battery of FIG. 1.

FIG. 3 is an example plot of charge current versus time.

FIG. 4 is an example plot of cell voltage versus time.

FIG. 5 is an example plot of cell resistance versus time.

DETAILED DESCRIPTION

Referring to FIG. 1, a vehicle 10 may include a traction battery 12, low voltage auxiliary battery 14, control module(s) and/or battery charger 16, and driver interface 17 (e.g., display screen/panel, speaker system, etc.) As known in the art, the traction battery 12 may be arranged to provide energy to move the vehicle 10; the auxiliary battery 14 may be arranged to provide energy to auxiliary loads such as lighting, etc.

The vehicle 10, in the embodiment of FIG. 1, is a plug-in hybrid electric vehicle (PHEV). (Other vehicle configurations such as battery electric, etc., however, are also contemplated.) The controller/charger 16, therefore, may be electrically connected with a power grid 18 (e.g., it may be plugged-in to a wall outlet) and permit energy to flow from the grid 18 to either of the batteries 12, 14 to charge the batteries 12, 14.

An auxiliary battery may be charged to a fixed voltage or a voltage that is a function of ambient temperature. If an auxiliary battery contains a cell with relatively low capacity (due to aging) or a cell that is shorted, an attempt to charge the battery to a fixed voltage may result in an overcharge condition of the low capacity cell or an overcharge condition of non-shorted cells. Overcharging can result in cell heating and release of hydrogen (gassing), which may adversely affect the auxiliary battery.

In a vehicle powered by an internal combustion engine, cell heating and/or gassing may be of little concern as the auxiliary battery is charged during driving. The effects of cell heating and/or gassing may be mitigated by the airflow experienced during driving. Additionally, because the auxiliary battery provides energy to start the engine, a weak or shorted cell is likely to result in poor or no engine starting. The auxiliary battery will likely be replaced before the heating or gassing condition occurs.

In alternatively powered vehicles, such as the PHEV 10, the auxiliary battery 14 does not provide starting energy. It may thus continue to be operated with a weak or shorted cell. Also, the power line from the grid 18 may have a limited amount of power it can provide to the controller/charger 16. Excessive charging of the auxiliary battery 14 can increase the amount of time (and cost) it takes to complete a vehicle charge.

An auxiliary battery may be found to be “good” or “bad” by connecting a relatively small resistance (e.g., 50 mΩ) across its terminals to draw a rather large load current. Under these circumstances, the voltage of a “good” 12 V battery, for example, may drop to 9 V while the voltage of a “bad” 12 V battery may drop to 4 V. This technique, however, cannot be performed during battery charge and may not reveal why the battery is “bad.”

Certain embodiments disclosed herein may assess a condition of the auxiliary battery 14 during charge, select a charge voltage and maximum current based on the condition, select a set point voltage and current based on the condition, and/or notify a vehicle operator of the condition and/or selections. The age and/or state of plate deterioration, for example, may be assessed based on the behavior of cell voltages when the auxiliary battery 14 is exposed to certain charge profiles. Once the condition of the auxiliary battery 14 is determined, the charge and operating profiles may be tailored to the specific conditions detected. A driver may also be informed, via the interface 17, as to the condition of the auxiliary battery 14 (e.g., age), the capability of the auxiliary battery 14 to hold charge, whether the auxiliary battery 14 has a bad cell, whether a modified charge profile has been implemented, whether a new battery is needed, etc. Other scenarios are also possible.

Referring to FIGS. 1 and 2, an equivalent circuit representation of the auxiliary battery 14 includes a state of charge (SOC) dependent voltage source, V_(SOC), a cell capacitance, C_(cell), incorporating both electrical and active material activation energy requirements, a chemical process resistance, R_(chem), a cell resistance, R_(cell), a leakage resistance, R_(leak), and terminals 20, 22. Pulses of generally constant current from the controller/charger 16, i_(chg), may be applied to the terminals 20, 22.

When the controller/charger 16 applies i_(chg), to the terminals 20, 22, C_(cell) will begin to acquire charge (until the activation energy component of C_(cell) is met, however, little energy will flow through R_(chem) and R_(leak)). Its voltage will thus change according to the equation

$\begin{matrix} {{v_{SOC}(t)} = {{\frac{1}{C_{cell}}{\int{\left( {i_{chg} - \frac{V_{SOC}}{R_{leak}}} \right){t}}}} + {i_{chg}*R_{cell}} + {V_{SOC}(0)}}} & (1) \end{matrix}$

When C_(cell) is charged, energy will flow through R_(chem) for storage in V_(SOC). The change in V_(SOC) during charge is proportional to the ratio of added energy stored in the auxiliary battery 14 to its total energy storage capability. In addition, the change in V_(SOC) is small compared to the total value of V_(SOC), resulting in the change in V_(SOC) being a linear representation of A−hrs stored during the charge. The equation for v_(SOC)(t) during this storage phase is

v _(SOC)(t)=i _(chg)*(R _(cell) +R _(chem))+V _(SOC)(0)  (2)

When the charge is stopped, i_(chg)=0 and C_(cell) will discharge into V_(SOC) through R_(chem) according to the equation

$\begin{matrix} {{v_{SOC}(t)} = {{\frac{1}{C_{cell}}{\int{i_{cell}{t}}}} = {{i_{cell}*R_{chem}} + V_{SOC}}}} & (3) \end{matrix}$

where i_(cell) is the current associated with C_(cell).

The controller/charger 16 (or other suitable controller) may monitor the above described change in voltage. As explained below, this change in voltage along with other parameters may be used to assess the condition of the auxiliary battery 14.

Referring to FIGS. 1 and 3, the controller/charger 16 may apply one or more current pulses to the auxiliary battery 14 using known techniques, such as those described in U.S. Pat. No. 3,857,087 to Jones. For example, the duration of the pulses of FIG. 3 are sufficiently long (e.g., 30 sec.) such that C_(cell) is able to charge and a small amount of energy is transferred into V_(SOC). Each pulse is followed by a rest period sufficiently long (e.g., 30 sec.) such that C_(cell) is able to discharge to the value of V_(SOC). Other suitable profiles, however, may be used.

Referring to FIGS. 3 and 4, the cell voltage, v_(SOC)(t), changes according to (1), (2) and (3) as the current pulses are applied and removed. Several voltages of interest, V₀, V₁, V₂, V₃, V₄, V₅, are labeled. These voltages along with the charge current magnitude and duration may be used to determine the following impedance parameters:

$\begin{matrix} {R_{cell} = \frac{\left( {V_{1} - V_{0}} \right)}{i_{chg}}} & (4) \\ {R_{chem} = {\left( \frac{\left( {V_{3} - V_{4}} \right)}{i_{{chg}@V_{3}}} \right) - R_{cell}}} & (5) \end{matrix}$

C_(cell) can be determined from R_(chem) and the rate of decay in voltage from V₄ to V₅. During this interval, (3) can be rewritten as

V _(SOC)=(V ₄ −V ₅)*e ^(−t/RC) +V ₅  (6)

or,

$\begin{matrix} {C_{cell} = \frac{- t}{R_{chem}*{\ln \left( \frac{V_{SOC} - V_{5}}{V_{4} - V_{5}} \right)}}} & (7) \end{matrix}$

(Of course, C_(cell) may also be calculated based on V₁ and V₂.) These impedance parameters change with the condition of the auxiliary battery 14 of FIG. 1 and may be used to determine how the auxiliary battery 14 should be charged as well as when it should be replaced.

A typical battery is rated in A−hrs for a charge/discharge cell voltage range. That is,

A−hrs=∫i _(chg) dt  (8)

The difference in A−hrs during a cell charge compared with a cell discharge is due to the electrical and electro-chemical parameters of the cell and will result in less A−hrs during discharge of a fixed A−hr charge due to the cell energy loss via R_(cell) and R_(chem). The ratio of A−hrs out to A−hrs in may be given by

$\begin{matrix} {{{Health\_ Term}\_ 1} = {\frac{I_{chg}*{time}}{\frac{V_{SOC}}{t}} = \frac{I_{chg}*{time}_{({V_{4} - V_{0}})}}{V_{5} - V_{0}}}} & (9) \end{matrix}$

The total A−hrs stored by the battery 14 may be given by

$\begin{matrix} {{{Health\_ Term}\_ 2} = \frac{A - {hrs}}{\left( {V_{5} - V_{0}} \right)*K}} & (10) \end{matrix}$

where K is a constant that may be determined via testing, etc.

Referring to FIGS. 1 and 5, the cell resistance, R_(cell), of the auxiliary battery 14 is expected to increase as it ages. This information may be determined in any suitable fashion including testing, simulation, etc. The controller/charger 16 may store information related to such an expected aging curve and compare determined values of R_(cell) against it to assess battery age/condition. For example, the controller/charger 16 may periodically (e.g., once each day) determine several R_(cell) values using the algorithms described above. The controller/charger 16 may average these R_(cell) values and place the average value along the aging curve according to the number of days that have passed since the battery was new (i.e., the first time the controller/charger 16 determined R_(cell)). As subsequent R_(cell) values are determined, they may be stored and used to determine whether the auxiliary battery 14 is at its end of life as discussed below.

The rate of change of R_(cell) over time (i.e., the slope of the curve illustrated in FIG. 5) becomes steeper as the auxiliary battery 14 approaches its end of life. A slope of at least a certain value may thus indicate that the auxiliary battery 14 is at its end of life. If the difference between a current value of R_(cell) and a previously recorded value of R_(cell) divided by the time passed between the two measurements is greater than a predetermined threshold (e.g., a slope that is 3 times greater than the slope around the new battery region), the controller/charger 16 may determine that the auxiliary battery 14 is at its end of life and needs to be replaced.

R_(cell) values below the allowable variation region of the aging curve are indicative of a new battery. (The allowable variation region, for example, may represent ±5% of a mean value, and be determined via testing, simulation, etc.) In these circumstances, the controller/charger 16 may reinitialize/clear its record of any stored R_(cell) values.

R_(cell) values above the allowable variation region of the aging curve may be indicative of battery defects. In these circumstances, the controller/charger 16 may determine the likely cause of the battery defects and whether the auxiliary battery 14 is recoverable based on, for example, the values of the parameters from (7), (9) and (10).

The C_(cell), Health_Term_1 and Health_Term_2 impedance parameters each have ranges of values that may be considered normal. That is, these parameters may be expected to take on values within these ranges under normal operating circumstances. Example normal ranges include, for C_(cell), the new battery C_(cell) value to 110% of that value, for Health_Term_1, 0.9 to 1.0, and for Health_Term_2, the new battery Health_Term_2 value to 60% of that value. Other ranges are also possible depending on the type of battery, etc.

Values less than the above ranges may be indicative of defect conditions (e.g., a shorted cell, battery dry out, a sulphated plate) as detailed in Table 1.

TABLE 1 Shorted Cell Dry Out Sulphation C_(cell) Normal Low Normal Health_Term_1 Low Normal Low Health_Term_2 Normal Normal Low

The extent to which a parameter is “Low” may determine whether a defective battery is recoverable. As an example, if any of the parameters are 50% to 99% of their lower threshold normal values, the battery may be considered recoverable (through application of a proper charging profile as explained below). If any of the parameters are less than 50% of their lower threshold normal values, the battery may be considered unrecoverable. These recoverable/unrecoverable ranges depend on the type of battery and other design considerations. As a result, they may be determined based on testing, simulation, etc.

To extend the life of the auxiliary battery 14, the controller/charger 16 may tailor charging and/or operating profiles for the auxiliary battery 14 based on the above information. A charge profile may be defined by a charge voltage and maximum current. Absent any of the above battery diagnosis information (or under normal operating circumstances), the controller/charger 16 may select a default charge voltage and default maximum current at which to charge the auxiliary battery 14. These default values, however, may be altered based on battery age. For example, the default charge voltage may be increased based on the battery age. (Herein, age may be defined by the time that has passed, e.g., 389 days, from a “new battery” determination or age may be defined by the value of the slope of the aging curve, etc.) If, for example, the battery is 400 days old, the charge voltage may be increased by 10% relative to the default (or “new battery”) charge voltage. Alternatively, if the current slope of the aging curve is 0.8, the charge voltage may be increased by 15% relative to the default charge voltage, etc. The optimum amount by which to alter the charge voltage may be determined via testing, simulation, etc.

The default charge parameters may also be altered based on whether the auxiliary battery 14 exhibits a defect as detailed in Table 2.

TABLE 2 Shorted Cell Dry Out Sulphation Charge Decrease Increase Increase Voltage Maximum No Change Decrease No Change Current If, for example, the controller/charger 16 detects a shorted battery cell, the charge voltage may be decreased 10% relative to the “new battery” charge voltage. If, for example, the controller/charger 16 detects a dry out condition, the age adjusted charge voltage may be increased by 12% and the maximum current may be decreased by 18%. If, for example, the controller/charger 16 detects a plate sulphation condition, the age adjusted charge voltage may be increased by 10%. Again, the optimum amount by which to alter the charge voltage and/or maximum current may be determined via testing, simulation, etc.

An operating profile may be defined by a set point voltage and set point current. Absent any of the above battery diagnosis information (or under normal operating circumstances), the controller/charger 16 may select a default set point voltage and set point current at which to operate the battery 14. These default values, however, may be altered based on battery age. That is, the set points are a function of battery age. For example, the default set point voltage and default set point current may be increased based on the battery age. The optimum amount by which to alter the set points may be determined via testing, simulation, etc.

The default operating parameters may also be altered based on whether the auxiliary battery 14 exhibits a defect as detailed in Table 3.

TABLE 3 Shorted Cell Dry Out Sulphation Set Point Decrease Increase Increase Voltage Set Point No Change No Change No Change Current If, for example, the controller/charger 16 detects a shorted cell, the set point voltage may be decreased by 6%. If, for example, the controller/charger 16 detects a dry out condition, the set point voltage may be increased by 5%. If, for example, the controller/charger 16 detects a plate sulphation condition, the set point voltage may be increased by 15%. Again, the optimum amount by which to alter the set points may be determined via testing, simulation, etc.

As apparent to those of ordinary skill, the algorithms disclosed herein may be deliverable to a processing device, which may include any existing electronic control unit or dedicated electronic control unit, in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The algorithms may also be implemented in a software executable object. Alternatively, the algorithms may be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), state machines, controllers or other hardware components or devices, or a combination of hardware, software and firmware components.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. 

1. A vehicle comprising: a battery; and at least one control module configured to apply a generally constant current to the battery for a period of time, to measure a response voltage of the battery to the applied current, to determine battery impedance parameters based on at least one of the applied current, period of time, and response voltage, and to determine a degradation condition of the battery based on the battery impedance parameters.
 2. The vehicle of claim 1 further comprising a driver interface, wherein the at least one control module is further configured to report the degradation condition of the battery via the driver interface.
 3. The vehicle of claim 1 wherein the battery impedance parameters include battery cell resistance.
 4. The vehicle of claim 3 wherein the at least one control module is further configured to record a history of battery cell resistance and wherein determining a degradation condition of the battery based on the battery impedance parameters includes determining a rate of change between a current battery cell resistance and a previously recorded battery cell resistance and comparing the determined rate of change with a predetermined rate of change to determine whether the battery is at its end of life.
 5. The vehicle of claim 3 wherein determining a degradation condition of the battery based on the battery impedance parameters includes determining whether the battery cell resistance falls within an expected range.
 6. The vehicle of claim 5 wherein determining a degradation condition of the battery based on the battery impedance parameters further includes determining whether other of the battery impedance parameters fall within a predefined range if the battery cell resistance is greater than the expected range to determine whether the battery is recoverable.
 7. The vehicle of claim 5 wherein the at least one control module is further configured to record a history of battery cell resistance and to clear the recorded history of battery cell resistance if the battery cell resistance is less than the expected range.
 8. The vehicle of claim 3 wherein determining a degradation condition of the battery based on the battery impedance parameters further includes comparing the determined battery cell resistance with an expected battery cell resistance to determine an age of the battery.
 9. The vehicle of claim 1 wherein the impedance parameters include battery plate capacitance.
 10. The vehicle of claim 1 wherein determining a degradation condition of the battery based on the battery impedance parameters includes determining whether any of the battery impedance parameters has a value less than a predefined range of values.
 11. A vehicle comprising: a battery disposed within the vehicle; and a battery charger disposed within the vehicle and configured to determine a degradation condition of the battery while operating to charge the battery.
 12. A method for diagnosing a battery comprising: applying a generally constant current to the battery for a period of time; measuring a response voltage of the battery to the applied current; determining battery impedance parameters based on at least one of the applied current, period of time, and response voltage; and determining a degradation condition of the battery based on the battery impedance parameters.
 13. The method of claim 12 further comprising reporting the degradation condition of the battery.
 14. The method of claim 12, wherein the battery impedance parameters include battery cell resistance, further comprising recording a history of battery cell resistance, and wherein determining a degradation condition of the battery based on the battery impedance parameters includes determining a rate of change between a current battery cell resistance and a previously recorded battery cell resistance and comparing the determined rate of change with a predetermined rate of change to determine whether the battery is at its end of life.
 15. The method of claim 12 wherein the battery impedance parameters include battery cell resistance and wherein determining a degradation condition of the battery based on the battery impedance parameters includes determining whether the battery cell resistance falls within an expected range.
 16. The method of claim 15 wherein determining a degradation condition of the battery based on the battery impedance parameters further includes determining whether other of the battery impedance parameters fall within a predefined range if the battery cell resistance is greater than the expected range to determine whether the battery is recoverable.
 17. The method of claim 15 further comprising recording a history of battery cell resistance and clearing the recorded history of battery cell resistance if the battery cell resistance is less than the expected range.
 18. The method of claim 12 wherein the battery impedance parameters include battery cell resistance and wherein determining a degradation condition of the battery based on the battery impedance parameters includes comparing the determined battery cell resistance with an expected battery cell resistance to determine an age of the battery.
 19. The method of claim 12 wherein the impedance parameters include battery plate capacitance.
 20. The method of claim 12 wherein determining a degradation condition of the battery based on the battery impedance parameters includes determining whether any of the battery impedance parameters has a value less than a predefined range of values. 